With reference to a partial enlarged cross sectional view of FIG. 12, an explanation will be made on a bonding pad electrode structure conventionally used. In FIG. 12, a reference numeral 101 designates an aluminum film electrically coupled with a predetermined region of a semiconductor substrate which is not shown in the drawing and in which a circuit element is formed. For example, if the circuit element is a MOSFET, the aluminum film 101 is a source electrode coupled with a source region of the MOSFET. On the aluminum film 101, there is formed a cover insulating film 102 made, for example, of PSG (phospho silicate glass). The cover insulating film 102 has an opening 102a. On a portion of the aluminum film 101 which is exposed via the opening 102a of the cover insulating film 102, there is formed a TiNiAg film 103. Ideally, the TiNiAg film 103 is formed such that it partially overlaps the cover insulating film 102. Thereby, it becomes possible to obtain a bonding pad electrode structure in which the aluminum film 101 can be protected from a corrosive substance which can corrode aluminum.
The above-mentioned conventional bonding pad electrode structure is fabricated as follows. That is, after forming the opening 102a in the cover insulating film 102, the TiNiAg film 103 is deposited on whole area of the semiconductor substrate. The TiNiAg film 103 comprises, for example, a Ti film having a thickness of 1000 angstroms, an Ni film having a thickness of 1000 angstroms and an Ag film having a thickness of 10000 angstroms. A photo resist film having predetermined patterns, i.e., a patterned photo resist film, not shown in the drawing, is formed on the TiNiAg film 103 such that the patterned photo resist film overlaps with the cover insulating film 102. In this case, a photolithography method is used. The patterned photo resist film masks an area wider than the area of the opening 102a of the cover insulating film 102. By using the patterned photo resist film as an etching mask, the TiNiAg film 103 is etched. The patterned photo resist film is then removed. Thereby, the bonding pad electrode structure shown in FIG. 12 can be formed. However, the process of etching the TiNiAg film 103 having the above-mentioned thickness is technically very difficult process and workability or productivity by the above-mentioned method is very low. Therefore, manufacturing costs of the semiconductor device become high.
In order to solve such problems of the above-mentioned method, an improved method is conventionally used. With reference to partial enlarged cross sectional views of FIG. 13A, FIG. 13B and FIGS. 14A-14C, an explanation will be made on such conventional improved method. In the first process, a cover insulating film 102 is formed on an aluminum film 101 by using an atmospheric pressure CVD method. The cover insulating film 102 is made of a PSG film having a thickness of, for example, 10000 angstroms. FIG. 13A is a cross sectional view showing a condition after finishing the first process.
After finishing the first process, in the second process, a patterned photo resist film 104 is formed on the cover insulating film 102 by using a photolithography method. The patterned photo resist film 104 has an opening 104a at a location corresponding to a bonding pad electrode. FIG. 13B is a cross sectional view showing a condition after finishing the second process.
After finishing the second process, in the third process, by using the patterned photo resist film 104 as an etching mask, a portion of the cover insulating film 102 at a location corresponding to the bonding pad electrode is removed. In this case, an isotropic etching method, such as an wet etching method, is used. Thereby, the opening 102a is formed in the cover insulating film 102. FIG. 14A is a cross sectional view showing a condition after finishing the third process.
After finishing the third process, in the fourth process, while leaving the patterned photo resist film 104, a TiNiAg film 103 is formed on the patterned photo resist film 104 by using a sputtering method. FIG. 14B is a cross sectional view showing a condition after finishing the fourth process.
After finishing the fourth process, in the fifth process, portions of the TiNiAg film 103 on the patterned photo resist film 104 are removed by using a lift-off method, and further the patterned photo resist film 104 is removed. FIG. 14C is a cross sectional view showing a condition after finishing the fifth process.
When the bonding pad electrode is fabricated by using the above-mentioned method illustrated in FIG. 13A, FIG. 13B and FIGS. 14A-14C, the following problems arise. That is, as shown in FIG. 14A, the cover insulating film 102 is side-etched and the opening 102a of the cover insulating film 102 becomes wider than the opening 104a of the patterned photo resist film 104. Therefore, as shown in FIG. 14B, on the aluminum film 101, a gap is produced between the TiNiAg film 103 and the cover insulating film 102. Thus, as shown in FIG. 14C, a portion of the aluminum film 101 is exposed via the gap between the TiNiAg film 103 and the cover insulating film 102. In such bonding pad electrode structure, it is impossible completely protect the aluminum film 101 from the corrosive substance.
In order to prevent the opening 102a of the cover insulating film 102 from becoming wider than the opening 104a of the patterned photo resist film 104 due to the side-etch of the cover insulating film 102, the following method can be considered. That is, by using the patterned photo resist film 104 as an etching mask, a portion of the cover insulating film 102 corresponding to a pad location can be removed by ion etching, such as plasma etching. However, in this case, another problem arises as follows. That is, there is a possibility that a portion of the TiNiAg film 103 on the patterned photo resist film 104 connects with a portion of the TiNiAg film 103 on the aluminum film 101. Thereby, it becomes difficult to remove portions of the TiNiAg film 103 on the patterned photo resist film 104 by using a lift-off method.